CLADDING TECHNOLOGY SHANXI CO., LTDCladding Repair Process for Medium-Speed Coal Mill Rollers
Literature Overview and Core Technical Content
The study "Cladding Repair Process for Medium-Speed Coal Mill Rollers" addresses a significant practical challenge in the power generation industry: the repair and restoration of worn coal mill rollers, which are critical components in the pulverizing of coal for combustion. Medium-speed coal mills are widely used in coal-fired power plants, and their rollers are subjected to severe abrasive wear, impact loading, and thermal cycling during operation. The rollers typically experience a wear rate of 0.5–2.0 mm per month, depending on the coal hardness and operating conditions, necessitating regular repair or replacement.
The literature investigates various cladding repair processes, including submerged arc welding (SAW) overlay, gas metal arc welding (GMAW) overlay, and flux-cored arc welding (FCAW) overlay, for restoring the geometry and wear resistance of coal mill rollers. The study examines the process parameters, microstructure, mechanical properties, and service performance of the cladding layers, with a focus on achieving a balance between wear resistance and impact resistance under the severe operating conditions of coal mills.
Roller Material and Wear Analysis
The base material of coal mill rollers is typically a low-alloy steel such as Q345R, 42CrMo, or 18CrNiMo, with a hardness of 200–300 HB. The surface of the roller is often hardened by induction hardening or flame hardening to a depth of 5–10 mm, achieving a hardness of 450–550 HV. However, this hardened layer is eventually worn away during service, exposing the softer base material to wear.
The wear mechanism in coal mill rollers is primarily abrasive wear, caused by the interaction between the roller surface and the coal particles. The coal particles, which can contain hard minerals such as quartz and feldspar, act as abrasives that progressively remove material from the roller surface. Impact wear is also a contributing factor, as the coal particles are accelerated and impact the roller surface at high velocities. The literature identifies that the wear rate is influenced by the coal hardness, particle size distribution, moisture content, and the operating speed of the mill.
Cladding Process Selection and Parameters
The literature evaluates several cladding processes for coal mill roller repair, with the following characteristics:
| Process | Deposition Rate | Hardness (HV) | Dilution | Suitability |
|---|---|---|---|---|
| SAW overlay | High (300–600 g/min) | 500–700 | 30–50% | Large area, thick overlay |
| GMAW overlay | Medium (150–300 g/min) | 400–600 | 25–40% | Flexible, repair |
| FCAW overlay | Medium-High (200–400 g/min) | 450–650 | 25–45% | Field repair, outdoor |
| PTA powder cladding | Medium (50–200 g/min) | 600–900 | 10–20% | Precision, multi-layer |
For coal mill roller repair, the SAW overlay process is generally preferred due to its high deposition rate and ability to build up thick overlay layers (5–15 mm) in a reasonable time. The welding parameters typically include a current of 400–600 A, a voltage of 30–40 V, and a travel speed of 200–400 mm/min. The welding consumable is typically a high-carbon, high-chromium steel wire such as D2MnSi or a similar grade, which produces a martensitic overlay layer with a hardness of 500–700 HV.
The literature also discusses the use of multi-layer cladding strategies to achieve a gradient in hardness and toughness. The first layer, deposited with a lower-carbon consumable, provides good bonding with the base material and reduces the risk of cracking. Subsequent layers, deposited with higher-carbon consumables, progressively increase the hardness and wear resistance of the overlay. This approach is particularly effective for rollers that are subjected to both abrasive and impact wear.
Microstructure and Mechanical Properties
The microstructure of the cladding layer on coal mill rollers is typically a tempered martensitic structure, with the hardness increasing from the base material interface to the surface. The literature reports that the hardness profile of a well-executed multi-layer SAW overlay can achieve a gradient from 300 HV at the interface to 650 HV at the surface, providing an optimal combination of toughness and wear resistance.
The mechanical properties of the overlay layer are critical for its performance in service. The literature reports the following typical values for a multi-layer SAW overlay on a coal mill roller:
| Property | Value |
|---|---|
| Hardness (surface) | 600–700 HV |
| Hardness (interface) | 300–400 HV |
| Tensile strength | 900–1200 MPa |
| Impact energy (Charpy V-notch, 20°C) | 20–40 J |
| Wear rate (ASTM G99) | 0.5–1.5 mg/N |
The impact energy of the overlay layer is a particularly important property, as it determines the resistance to spalling under impact loading. The literature emphasizes that the impact energy should be maintained above 20 J to prevent brittle fracture during operation. This is achieved by controlling the carbon equivalent of the overlay material and applying appropriate post-weld heat treatment.
Defect Analysis and Countermeasures
The literature identifies several common defects in the cladding repair of coal mill rollers:
- Cracking: Cracks can occur in the overlay layer or at the interface due to high residual stresses and the high carbon equivalent of the overlay material. Countermeasures include preheating the roller to 200–300°C, using a low-hydrogen flux, and applying post-weld heat treatment at 550–650°C for 2–4 hours.
- Porosity: Gas porosity can occur due to inadequate shielding or contamination of the flux. Countermeasures include ensuring proper flux drying, using a consistent welding current, and maintaining a clean welding area.
- Lack of fusion: Incomplete fusion between layers can occur due to excessive travel speed or insufficient current. Countermeasures include adjusting the welding parameters and ensuring proper preparation of the previous layer.
- Hardness variation: Uneven hardness across the overlay layer can occur due to inconsistent powder feed or welding parameters. Countermeasures include calibrating the equipment and using a multi-layer strategy with controlled inter-pass temperatures.
Engineering Practice and Service Performance
The literature presents case studies of cladding repair applied to coal mill rollers in power plants. In one case, a worn roller with a material loss of 8 mm was repaired using a multi-layer SAW overlay, achieving a surface hardness of 650 HV and a service life of 14 months before the next repair. In another case, a roller was repaired using PTA powder cladding with a WC-reinforced powder, achieving a surface hardness of 850 HV and a service life of 22 months.
These cases demonstrate that the selection of the appropriate cladding process and consumable is critical for achieving optimal service performance. The SAW overlay process is suitable for large-scale repair of heavily worn rollers, while the PTA powder cladding process is preferred for precision repair of lightly worn rollers where high hardness is required.
Study Insights and Reflections
The literature provides valuable insights into the practical challenges of cladding repair for coal mill rollers. One key finding is that the service life of the overlay layer is heavily influenced by the operating conditions of the coal mill, including the coal hardness, moisture content, and mill speed. The literature recommends that the overlay material and process be selected based on a thorough understanding of the specific operating conditions, rather than relying on generic recommendations.
Another important insight is the recognition that the residual stress state of the overlay layer is a critical factor in determining its service performance. High tensile residual stresses can initiate cracks under cyclic loading, leading to premature failure. The literature recommends that post-weld heat treatment be applied to all cladding repairs, even when it is not strictly required by the applicable standard. This practice, while adding to the repair cost, can significantly extend the service life of the overlay layer.
In conclusion, the study offers a comprehensive understanding of the cladding repair process for medium-speed coal mill rollers. The detailed analysis of process parameters, microstructure, mechanical properties, and service performance provides engineers with the knowledge needed to design and optimize repair strategies for specific applications. The practical case studies and process recommendations are directly applicable to industrial settings, offering a pathway to significantly extend the service life of coal mill rollers through strategic cladding repair.